In recent years, studies have been conducted on a system in which a base station device and a wireless device are connected by an optical fiber by using a technology called, for example, Radio on Fiber (RoF). In RoF, a wireless transmission signal is optically modulated by the base station device, is transmitted by the optical fiber to the wireless device placed in remote site, is amplified by the wireless device, and is then wirelessly transmitted. Accordingly, for example, it is possible to configure a system in which multiple wireless devices are connected to a single base station device, processes to be performed on a signal are collectively performed by the base station device, and the signal is wirelessly transmitted from each of the wireless devices. Consequently, the overall cost of a wireless communication system can be reduced.
In general, an amplifier is provided in a RoF wireless device in order to amplify a signal and then the signal is wirelessly transmitted. Normally, if an input level to an amplifier is relatively small, the linearity of the input and output signals is maintained, whereas, if an input level to an amplifier increases and an output level approaches the saturation level, the input and output of signals become nonlinear due to inter modulation distortion. Consequently, when an amplifier is operated at around the saturation level in which the power efficiency is high, the amplifier needs to be operated in a nonlinear region in which the input and output of signals become nonlinear.
When an amplifier is operated in a nonlinear region, it is preferable that inter modulation distortion generated in the amplifier be compensated. Specifically, by compensating inter modulation distortion that is generated at the time of amplification, it is possible to maximize the performance of the amplifier. In recent years, a predistortion method is often used, as a distortion compensation method, in which a distortion component that cancels out the inter modulation distortion generated in the amplifier is previously given to a transmission signal.
Patent document 1: Japanese Laid-open Patent Publication No. 2014-103571
However, if the predistortion method is used in the RoF system, there is a problem in that the distortion component added to the transmission signal is masked and thus it is difficult to sufficiently perform distortion compensation.
Specifically, if the predistortion method is used in the RoF system, a distortion component associated with inter modulation distortion generated in the amplifier is added to a transmission signal in the base station device. Then, the transmission signal is transmitted to a wireless device by using an optical fiber; however, the transmission by using the optical fiber brings noise, such as thermal noise, shot noise of a photodiode, relative intensity noise (RIN) of a light source. Accordingly, because noise is added to the transmission signal when the transmission signal is transmitted to the wireless device by the optical fiber, a relatively small level of a distortion component is masked by the noise.
Consequently, when the transmission signal is amplified by the amplifier, the inter modulation distortion is not sufficiently compensated and thus the degradation of adjacent channel leakage ratio (ACLR) occurs.